Microcapsules are small particles of solids, or droplets of liquids, inside a thin coating of a shell material such as starch, gelatine, lipids, polysaccharides, wax or polyacrylic acids. They are used, for example, to prepare liquids as free-flowing powders or compressed solids, to separate reactive materials, to reduce toxicity, to protect against oxidation and/or to control the rate of release of a substance such as an enzyme, flavour, a nutrient, a drug, etc.
Ideally, a microcapsule would have good mechanical strength (e.g. resistance to rupture) and the microcapsule shell would provide a good barrier to oxidation, etc.
A typical approach to meeting these requirements is to increase the thickness of the microcapsule wall. But this results in an undesirable reduction in the loading capacity of the microcapsule. That is, the “payload” of the microcapsule, being the mass of the loading substance encapsulated in the microcapsule divided by the total mass of the microcapsule, is low. The typical payload of such “single-core” microcapsules made by spray drying an emulsion is in the range of about 25-50%.
Another approach to the problem has been to create what are known as “multi-core” microcapsules. These microcapsules are usually formed by spray drying an emulsion of core material such that the shell material coats individual particles of core material, which then aggregate and form a cluster. A typical multi-core microcapsule is depicted in prior art FIG. 1. Multi-core microcapsule 10 contains a plurality of cores 12. The cores 12 take the form of entrapped particles of solids or of liquid droplets dispersed throughout a relatively continuous matrix of shell material 14. As a result, there is a high ratio of shell material to loading material and the payload of the multi-core microcapsule is therefore low. Moreover, despite the high ratio of shell material to loading substance in such microcapsules, the shell material is poorly distributed. As shown in prior art FIG. 1, many of the cores 12 are very close to the surface 16 of the microcapsule. The cores at the surface are therefore not well protected against rupture or from oxidation.
Known microcapsules therefore either have a poor payload, or fail to adequately contain and protect the loading substance deposited therein. Moreover, because these microcapsules are generally prepared in a single step, it is difficult to incorporate multiple functionalities, such as oxidation resistance, moisture resistance and taste masking into a single microcapsule.